In both systems of land-line and radio telephones, a user communicates by means of a handset having a speaker at its one end to be placed close to the user's ear and a microphone at its other end to be applied to the user's mouth. Thus, only one hand of the user is free since the other hand of the user must be used to hold the handset. A speaker phone has been developed in order to provide the user with hands-free operation. The speakerphone is generally coupled to a telephone line and designed for the user to be able to move freely while communicating with another party. The speakerphone is generally used to compare the volume of two users and select the larger one of the two volumes, and such a conventional speaker phone is well operated on a land-line telephone system but may not be operated correctly on a radio telephone system in a high level of ambient background noise such as in a vehicle. For example, the speakerphone in the high level of ambient background noise has a possibility to always select an audio sound from the hands-free party and block all communication signals from the other party.
As shown in FIG. 1, a circuit of a conventional hands-free radio telephone is introduced for solving such a problem as blocking communications in a high level of ambient background noise. The circuit was applied to the patent office of the Republic of Korea for a patent by a semiconductor manufacturer, Motorola Co. of the U.S.A. on Mar. 25, 1982 and granted with the patent No. 88-1016 published in the patent gazette No. 1406 of Jun. 13, 1988.
Referring to FIG. 1, the circuitry comprises a microphone 12 to convert an audio signal into an electric signal, a speaker 32 to convert an electric signal into an audio signal, a plurality of amplifiers 14, 18, 26, and 30 to process the electric signal with a given value in order to connect them with a public telecommunication network, a first detection circuit 34 to detect the presence of a signal from the microphone 12, a second detection circuit 36 to detect the presence of a signal from the public telecommunication network, a control circuit 38 to generate a control signal for switching a transmission path and/or a reception path of outputs of the first and/or the second detection circuits 34, 36, audio switches 16, 28 to transmit or block the audio signal according to the control signal, a transmission signal processor 20 to process the signal from the microphone 12 passed through the amplifier 14, the audio switch 16 and the amplifier 18 for connecting them with the public telecommunication network, a transmission/reception separating circuit 22 to connect the transmission and the reception signals with a public telecommunication network, and a reception signal processor 24 to suitably treat the reception signal separated from the transmission/reception separating circuit 22 and transmit the resultant signal to the reception path.
All the details of the circuit described on the patent No. 88-1016 in the patent gazette No. 1406 will be summarized as follows. An electric audio signal generated from a microphone 12 is passed through an amplifier 14 and transmitted to a first detection circuit 34 and an audio switch 16. On the other hand, a reception signal from a public telecommunication network 42 is passed through a transmission/reception separating circuit 22 and a reception signal processor 24, and amplified to a given level by an amplifier 26. The amplified signal from the amplifier 26 is transmitted to a second detection circuit 36 and an audio switch 28. The first detection circuit 34 has a first predetermined state in the presence of the audio signal and a second predetermined state in the absence of the audio signal by detecting the signals from the microphone 12. And the second detection circuit 36 has the first predetermined state in the presence of the audio signal and the second predetermined state in the absence of the audio signal by detecting the reception signals from the public telecommunication network 42. A control circuit 38 generates a control signal of the first predetermined state in the presence of the reception signal and the second predetermined state in the presence of the transmission signal by examining the signals from the first and the second detection circuits 34, 36. A reception audio switch 28 is closed to pass the reception audio signal and the transmission audio switch 16 is opened by an inverting gate 40 to block the transmission of an audio signal in the presence of the control signal of the first predetermined state from the control circuit 38. On the contrary, the transmission audio switch 16 is closed to pass the transmission audio signal and the reception audio switch 28 is opened to interrupt the reception audio signal, in the presence of the control signal of the second predetermined state from the control circuit 38. On account of the audio switches 16, 28 of the conventional circuit as shown in FIG. 1 being able to do only two actions of closing and opening, the audio switches 16, 28 make some syllables intermittent and do not bring about a smooth communication when two users speak simultaneously or they are in an ambient background noise, and therefore it is very difficult to maintain the performance when applying the circuit to land-line and radio telephone systems. In the conventional circuit as described above, it is impossible to perform a full duplex communication because the control circuit 38 is activated to close one audio switch 16 (or 28) and open simultaneously the other audio switch 28 (or 16) for transmitting a certain audio signal to a certain path.